Method and apparatus for reducing contamination in a wafer loadlock of a semiconductor wafer processing system

ABSTRACT

A method and apparatus for heating a loadlock to inhibit the formation of contaminants within the loadlock. At least one heater is attached to the walls of the loadlock to boil contaminants from the surfaces within the loadlock. These desorbed contaminants are exhausted from the loadlock by a vacuum pump. Alternatively, a purge gas can be supplied to the loadlock while the loadlock is being heated. The flow of purge gas flushes the desorbed contaminants from the loadlock.

BACKGROUND OF THE DISCLOSURE

1. Field of the Invention

The invention relates to semiconductor wafer process systems and, moreparticularly, the invention relates to a method and apparatus forcontrolling contamination in a loadlock of a semiconductor waferprocessing system.

2. Description of the Background Art

Semiconductor wafer processing systems comprise a loadlock wherein aplurality of wafers are stacked that are awaiting processing within asystem. These wafers are removed from the loadlock one at a time by arobot and transported to various processing chambers within the system.Once processed, the wafers are returned from the process chambers to thewafer cassette in the loadlock for removal from the system.

During wafer processing within the system contaminants adsorb onto thewafers. Typically the reactant gases adsorb onto the wafer surface andwhen the wafer is returned to the loadlock the adsorbed material willdesorb. The desorbed gases combine with moisture in the loadlock to forma corrosive film that coats the interior surfaces of the loadlock andthe wafers. Such coating of the interior surfaces causes corrosion ofthe surfaces within the loadlock, and causes the formation ofcondensation particles upon the wafers. The surface corrosion createstremendous quantities of corrosion byproduct particulates that dispersethroughout the loadlock to contaminate the wafers.

Therefore, a need exists in the art for a method and apparatus thatcontrols corrosive contaminants within a loadlock.

SUMMARY OF THE INVENTION

The disadvantages associated with the prior art are overcome by a methodand apparatus that heats the atmosphere of a loadlock. Specifically, theapparatus heats the loadlock to inhibit the formation of corrosivebyproduct particles. In addition, the apparatus may supply a purge gasto the loadlock to dilute and remove both moisture and corrosive gasesfrom the loadlock. To provide heat to the loadlock, at least one heateris attached to the walls of the loadlock to desorb the contaminants fromthe surfaces within the loadlock. These desorbed contaminants areexhausted from the loadlock by a vacuum pump or flushed from theloadlock by a flow of the purge gas. As such, a combination of heatingand purging effectively eliminates both the moisture and corrosive gasesfrom the loadlock to eliminate a source of wafer contamination.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts a semiconductor processing system including apparatus forremoving contaminants from one or more loadlocks;

FIG. 2 depicts a perspective view of a loadlock having a plurality ofblanket heaters attached to the exterior surfaces of loadlock; and

FIG. 3 depicts a cross-sectional view of the loadlock of FIG. 2 takenalong line 3—3;

FIG. 4 depicts a schematic design of a heater arrangement; and

FIG. 5 depicts a flow diagram representing operation of the presentinvention.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

FIG. 1 depicts a semiconductor wafer processing system 100 comprising asystem hardware 102 coupled to a loadlock contaminant control system 104of the present invention. The system hardware operates in a manner thatis generally known in the art while the contaminant control systemprovides a unique technique for heating and exhausting an atmosphere inthe loadlock(s) to eliminate a source of wafer contamination. A purgegas may be supplied to the loadlock during heating.

Specifically, the wafer processing hardware 102 comprises a plurality ofprocess chambers 112 and 110, and a pair of loadlocks 122 and 124coupled to a central platform 113 that contains a transfer chamber 103.Within transfer chamber 104 is a robot 106 having a blade 108 located atthe distal of the robot arms and adapted for carrying semiconductorwafers from one process chamber to another and between the loadlocks andthe process chambers. The robot's blade 108 has access to the chambers112 and 110 through respective slit valves 114 and 116. The platform isalso coupled to at least one loadlock 122. In the depicted platformthere are two loadlocks 122 and 124. Each of the loadlocks 122 and 124are respectively coupled to the transfer chamber 104 via slit valves 118and 120.

In operation, wafers 128 and 126 within the loadlocks are accessed bythe robot's blade 108 through the respective slit valves 118 and 124.The wafers are carried to a particular process chamber 112 or 110wherein they are processed and then returned to the loadlock for removalfrom the system. Typically corrosives and other processing byproductsadsorb upon the wafers as they are processed within chambers 112 and 110and the loadlock contamination control system 104 removes thesecorrosives. As such, the corrosives are prevented from attacking theinterior surfaces of the loadlocks and combining with loadlock moistureto form condensation particles on unprocessed wafers in the loadlock.

In one embodiment of the invention, the contamination control system 104comprises a gas input subsystem 160, a gas exhaust subsystem 162 and aloadlock heating subsystem 164. The gas input subsystem 160 is optional.The gas input subsystem 160 comprises a gas source 140, a plurality ofvalves 136A, 136B and 136C, and a plurality of set screws 138A and 138B.The exhaust subsystem comprises a pair of valves 134A and 134B and apump 144. The heater subsystem comprises a heater controller 146, athermocouple 130 and at least one heater element 132 that is attached orembedded in the side wall or side walls of the loadlock 122 and/or 124.

In operation, the gas source 140 supplies an inert gas such as nitrogenthrough valves 136C through the set screws 138A and 138B and valves 136Aand 136B to the loadlocks 122 and 124. The set screws 138A and 138B areneedle valves that, upon a system initialization, are used to set theflow rates into the chamber such that the flow of gas is balancedbetween one loadlock and the other such that the pressure within theloadlocks is in the correct regime for efficient removal of moisture andcorrosives. The plurality of valves 136A, 136B and 136C are used tocontrol the flow of gas to the respective loadlocks such that the gascan be decoupled from a loadlock that is being opened to remove or addadditional wafers to the loadlock.

The pumping system comprises a pair of exhaust valves 134A and 134B thatare coupled to a manifold 135 that carries the exhaust gases to the pump144. In this manner the inert gas is supplied to the loadlock, flowsthrough the loadlock causing contaminants to be removed from theloadlock via the gas flow to the pump 144. The gas flow is maintained atapproximately 250 sccm where a pressure of 400—500 mT is maintainedwithin each of the loadlocks.

To inhibit the formation of corrosive particles on loadlock surfaces, atleast one heater element 132 is attached or embedded in the side wall ofeach of the loadlocks 122 and 124. A heater controller applies electriccurrent to the heater element to heat the interior gas in the loadlock122. The interior of the loadlock is maintained at approximately 50-55°C. or more. To facilitate dynamic control of the heating process atleast one thermocouple 130 is attached to the loadlock wall. The outputvoltage from the thermocouple 130 is coupled to the heater controller146 which, in response to the signal from the thermocouple, modifies thevoltage applied to the heater to maintain a constant temperature withinthe loadlock. The temperature change from top to bottom within theloadlock is approximately 5-6° C. To facilitate this stringenttemperature differential the heater controller 146 is used to control aplurality of zones of heater elements and a plurality of thermocouplesare used to provide feedback voltage with respect to each zone. Adetailed description of the zonal heater control system is provided withrespect to FIG. 4.

The contaminant control system 104 comprises a controller 150 which mayform part of the wafer processing system controller 148. The controller150 comprises a central processing unit (CPU) 152, a memory 158, supportcircuits 156 and input/output (I/O) circuits 154. The CPU 152 is ageneral purpose computer which when programmed by executing software 159contained in memory 158 becomes a specific purpose computer forcontrolling the hardware components of the contaminant control system104. The memory 158 may comprise read only memory, random access memory,removable storage, a hard disk drive, or any form of digital memorydevice. The I/O circuits comprise well known displays for output ofinformation and keyboards, mouse, track ball, or input of information.The support circuits 156 are well known in the art and include circuitssuch as cache, clocks, power supplies, and the like.

The memory 158 contains control software 159 that when executed by theCPU 152 enables the controller to digitally control the variouscomponents of the contaminant control system 104. A detailed descriptionof the process that is implemented by the control software is describedwith respect to FIG. 5.

Although the heater controller 146 is generally autonomous, the heatercontroller 146 provides the controller with fault and error informationregarding the heater operation. Alternatively, the heater controller maybe a portion of the controller 150 of the digital system. In fact, thecontroller 148 of the semiconductor wafer processing system 102 thatcontrols the processes that occur within system 102 as well as thecontaminant control system 104 may also incorporate the heatercontroller 146 as depicted by the dashed box that circumscribes thecontroller 150 as well as the heater controller 146.

FIG. 2 depicts a perspective view of an individual loadlock 122, whileFIG. 3 depicts a cross-sectional view of the loadlock 122 taken alonglines 3—3 in FIG. 2. FIGS. 2 and 3 should be referred to simultaneouslyto best understand the invention.

Loadlock 122 comprises a top 122T, a bottom 122B, and four sides122S₁-122S₄. The side 122S₄ contains an aperture that is covered by adoor 200. The door 200, the top side 122T, the bottom side 122B as wellas sides 122S₁ and 122S₃ have attached thereto a heater element 132. Theheater elements 132 in the embodiment depicted are self-adhesiveresistive blanket heaters. Alternatively, the resistive blanket heaters132 can be replaced by embedded heater cartridges as well as conduitscarrying heated fluid. Other external heaters such as infra-red lampsare also considered within the scope of the invention. The heaters arerequired to heat the internal atmosphere of the loadlock 122 to atemperature that will desorb the contaminants that are contained withinthe loadlock. An adsorbed molecule of corrosive gas (represented atreference 310) is desorbed by the heating of the atmosphere within theloadlock 122 and exhausted from the loadlock by the purge gas flow. Atypical adsorbed material comprises hydrogen bromide (HBr) and isdesorbed by a temperature of 50-55° C. or more.

Gas is provided through a porous ceramic element 204. The element 204comprises an electro-polished stainless steel mounting flange 300 and analumina portion 302 having a 0.5 micron pore size. The ceramic element204 is mounted to the side wall 122S₃ via the flange 300 and a conduitcarrying the inert gas is coupled to the element 204. The flange 300 issealed to the wall 122S₃. The gas enters the chamber and is dispersed bythe ceramic element such that the gas does not enter at a high velocityand the gas is distributed through the wafers 308 contained in the wafercassette 306. To ensure that recondensation of corrosives does not occurin the exhaust manifold, heater elements may be placed on the conduitsthat lead to the pump to maintain the conduits at elevated temperatures.

FIG. 4 depicts a schematic diagram of the heating system 164 comprisingthe heater controller 146 as well a plurality of heating zone circuitry400, 402 and 404. Each zone comprises a thermocouple 130 ₁, 130 ₂ and130 ₃ and a heating element 132 ₁, 132 ₂ and 132 ₃. Any given zone maycomprise multiple heating elements such that multiple regions of theloadlock are heated in response to one or more thermocouple signals. Forexample, zone 400 may comprise a thermocouple on one side of theloadlock and heating elements on sides 122S₁, 122S₂ and 122S₄. While asecond zone 402 may comprise a thermocouple 130 ₂ on the door 200 and aheating pad 132 also located on the door. The third zone may comprise athermocouple on the top 122T of the chamber and a heating pad 132located on the top. Each zone is independently controlled to adjust thetemperature such that an attempt is made to uniformly heat theatmosphere within the loadlock. Through use of a standard feedbackcircuit to monitor a voltage that is generated with respect to thetemperature of the thermocouple, the current driven to the heater iscontrolled. As such, the temperature throughout the loadlock is helduniform to within plus or minus 5° C. while the overall temperature isabout 50° C. Higher temperatures may also be used.

When two chambers are simultaneously used as shown in FIG. 1, the valveassemblies are used to enable one loadlock to be used for supplyingwafers to the hardware while the second loadlock is open to atmosphere.As such, any combination of venting and pumping between the two chambers122 and 124 can be provided. With the selective opening and closing ofthe valves, the system of the present invention avoids backstreaming ofgases from one chamber to another.

FIG. 5 depicts a flow diagram of a process used by the invention. Thisprocess provides any combination of pumping and venting either or bothloadlocks 122 and 124 of FIG. 1. The process 550 is implemented byexecuting control software 159 upon CPU 152. The process 550 begins withthe system 104 in an initial state where the loadlocks 122 and 124 areopen, valves 136A, B, C are closed and all the heaters are active. Atstep 502, a cassette of wafers is placed in the loadlock 122 and thedoor is closed when the loadlock issues a “LOAD/UNLOAD” command. At step504, the valve 134A is opened. Ag step 506, the routine queries whetherthe pressure in loadlock 122 (P₁₂₂) is less than the base loadlockpressure (P_(B)). When the loadlock pressure attains the base pressure,the routine proceeds to step 508. At step 508, the valves 136A and 136Care opened and the loadlock 122 is evacuated to a nominal pressure of400-500 mT. At step 510, as the gas and heat remove contaminants, thewafers are transferred one by one into and out of the wafer processinghardware 102. At step 512, the process queries whether the secondloadlock 124 is to be used. Generally, this query is answered by acassette being placed in loadlock 124 and a “LOAD” button beingdepressed. If the LOAD request is not made, the process ends at step514. If the LOAD request is made, the process 550 proceeds to step 516.

At step 516, valve 136C is closed to temporarily stop the flow of inertgas. Then, at step 518, valve 136A is closed to isolate the loadlocksfrom one another. A delay of about one second occurs at step 520 before,at step 522, the valve 134A is closed to isolate the pump from loadlock122. After a delay of about one second occurs at step 524, step 526opens valve 134B. At step 528, the routine queries whether the pressurein loadlock 124 is less than the pressure in loadlock 122. When thepressure in loadlock 124 (P₁₂₄) is greater than or equal to the pressurein loadlock 122 (P₁₂₂), the routine proceeds to step 530. Then, at step530, the valve 134A is opened to pump the loadlock 124 to 400-500 mT. Atstep 531, valves 136A and 136B are opened. Then, at step 532, theroutine waits for a delay of about one second. To apply inert purge gas,valve 136C is opened at step 534 and the process 550 ends at step 536.At this time, both loadlocks 550 are being heated and purged ofcontaminants.

To unload a wafer cassette, an operator generally depresses an “UNLOAD”button corresponding to one of the loadlocks, e.g., loadlock 122. Anautomatic unload sequence may also be executed by the software. Ineither instance, the valve 136A is closed, then valve 134A is closed.The loadlock atmosphere is then vented to atmospheric pressure withnitrogen. In this manner either loadlock can be isolated from thecontaminant control system to allow a cassette to be removed, while theother loadlock is used. Once a new cassette is loaded, the loadlock 122can be pumped and purged using steps 516 through 536 of process 550;however valve 136B is substituted for 136A and valves 134B issubstituted for 134A and so on. Also, to unload loadlock 124, theprocess described above for unloading loadlock 122 can be used, exceptvalves 134B and 136B are used to isolate loadlock 124.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

What is claimed is:
 1. A method for controlling contaminants in a pairof loadlocks comprising the steps of: heating a first loadlock whilesimultaneously flowing purge gas through said first loadlock, where anatmosphere of said first loadlock is at a first pressure; stopping theflow of purge gas to said first loadlock by isolating said firstloadlock from a source of purge gas and a vacuum pump; isolating saidfirst loadlock from said second load lock; heating said second loadlockwhile simultaneously flowing purge gas through said second loadlock,where an atmosphere of said second loadlock in at a second pressure; andwhen said first pressure and said second pressure are the same,connecting said first chamber to said vacuum pump and said source ofpurge gas.
 2. The method of claim 1 wherein said purge gas is an inertgas.
 3. The method of claim 2 wherein the inert gas is nitrogen.
 4. Themethod of claim 1 wherein said heating step heats the atmosphere withineach of the loadlocks to about 50° C.
 5. The method of claim 1 furthercomprising the step of: isolating a select loadlock from said purge gassource and said vacuum pump; and venting an atmosphere of said selectloadlock.
 6. The method of claim 1 wherein said heating inhibits acorrosive reaction within said loadlocks.
 7. The method of claim 1wherein said heating is independently applied to a plurality of heatingzones.
 8. The method of claim 1 wherein said heating inhibits theformation of corrosion particles within said loadlocks.
 9. The method ofclaim 1 wherein a temperature of said loadlocks resulting from saidheating is dynamically controlled.